Okay, let's tackle this question. The patient is a 26-day-old infant with recurrent nonbilious vomiting, constipation, and weight loss. The key here is to consider the possible causes of these symptoms in a newborn. Nonbilious vomiting suggests the obstruction is proximal to the ampulla of Vater, so it's not something like biliary atresia. But the other symptoms are constipation and weight loss, which might point to a different issue.

First, I need to think about what causes these symptoms in neonates. Common conditions include pyloric stenosis, Hirschsprung's disease, and maybe other gastrointestinal obstructions. Pyloric stenosis typically presents with projectile vomiting, but it's usually around 2-8 weeks of age. However, pyloric stenosis doesn't usually cause constipation because the obstruction is in the stomach, so the baby might have normal bowel movements. Hirschsprung's disease, on the other hand, is a congenital absence of ganglion cells in the colon, leading to chronic constipation, failure to thrive, and sometimes vomiting due to bowel obstruction. Since the infant is only 26 days old, Hirschsprung's is a strong possibility here.

Another possibility is intestinal obstruction, but that's often associated with bilious vomiting. The nonbilious aspect makes that less likely. Maybe a meconium plug or meconium ileus, but those are usually in the first few days. The weight loss and constipation in this case make me think of Hirschsprung's. Also, the presence of nonbilious vomiting could be due to secondary obstruction from the constipation caused by the aganglionic segment.

So the correct answer is Hirschsprung's disease. Let me check the options again. The options aren't listed, but the correct answer is likely Hirschsprung's disease. Let me confirm the key points: Hirschsprung's presents with constipation from birth, failure to pass meconium, and later vomiting. The absence of ganglion cells leads to a functional obstruction. The other options like pyloric stenosis would have projectile vomiting and not constipation. Intestinal atresia would have bilious vomiting. So Hirschsprung's is the most likely diagnosis here.

**Core Concept**
This question assesses the differential diagnosis of gastrointestinal obstruction in neonates, focusing on **Hirschsprung’s disease**, a congenital aganglionic megacolon caused by the absence of ganglion cells in the distal colon. Key features include chronic constipation, nonbilious vomiting, and failure to thrive.

**Why the Correct Answer is Right**
Hirschsprung’s disease results from a failure of neural crest cells to migrate to the distal bowel, leading to **aganglionic segments** in the colon (most commonly the rectosigmoid region). The affected bowel cannot relax, causing functional obstruction. This leads to proximal bowel dilation, constipation, and secondary nonbilious vomiting due to colonic distension. Weight loss occurs from malabsorption and reduced oral intake. Diagnosis is confirmed via rectal biopsy showing absent ganglion cells.

**Why Each Wrong Option is Incorrect**
**Option A:** *Pyloric stenosis* presents with **projectile bilious vomiting** (though nonbilious in early stages) and

✓ Correct Answer: D. Pyloric stenosis